1. Technical Field
The present invention relates to a liquid crystal display, and in particular to a liquid crystal display which comprises a sealing material for sealing the peripheral portion in two substrates, and an end-sealing material for sealing the injection hole for liquid crystal, and a method for manufacturing such a liquid crystal display.
2. Discussion of Related Art
In an active matrix liquid crystal display (LCD) using thin film transistors, liquid crystal is sealed between a TFT array substrate, which has gate electrodes (Y-electrodes) and data electrodes (x-electrodes) arranged in a matrix and has thin film transistors (TFT) placed at the intersection points of them and an opposing substrate overlaid with a gap between the TFT array substrate, and the voltage applied to the liquid crystal is controlled by the thin film transistors to enable display by using the electrooptic effect of the liquid crystal.
To seal liquid crystal between two substrates of glass or the like, and protect the liquid crystal from external pollution such as moisture or environmental change, a sealing material is generally used. The sealing material is made of a thermosetting resin or an ultraviolet-curing resin, and it is formed in the peripheral portion of one substrate using screen printing or a writing method by dispenser. After placing the other substrate on one substrate on which the sealing material is formed, they are pressed and heated to a fixed degree, and if the sealing material is an ultraviolet-curing resin, it is cured by ultraviolet rays, thereby to bond the two substrates by means of the sealing material. For the sealing material, high mechanical bonding strength and high stability to change in environment such as temperature or moisture, as well as low curing temperature and no pollution of the liquid crystal by the curing agent are required as its characteristics. Further, in part of the sealing material, there is provided an opening, which is an injection hole for injection of the liquid crystal.
After the two substrates are bonded together by the sealing material, the closed area formed by the sealing material is evacuated, and the liquid crystal is injected from the injection hole. Thereafter, as the end-sealing material, an ultraviolet-curing resin is charged, and exposed to UV rays to seal the liquid crystal.
As solutions to various problems occurring in the injection of the liquid crystal, there are Published Unexamined Patent Applications No. 6-34984, 9-90330, and 61-45225. In the Published Unexamined Patent Application No. 6-34984, there is disclosed a technique for suppressing the velocity of liquid crystal by providing a protrusion and a wall in the injection hole, thereby to prevent damage of the alignment layer due to the movement of spacers. Further, in the Published Unexamined Patent Application No. 9-90380, there is disclosed a technique for doubly sealing the liquid crystal cell (LCD cell) by providing a second horn, which is a member for preventing the mixing of air outside the injection hole horn, thereby to prevent the external mixing of air by air bubbles. Furthermore, in the Published Unexamined Patent Application No. 61-45225, there is disclosed a technique in which a gap control material of the same material as the sealing material is provided inside the injection hole to divide the injection hole into a plurality of portions for stabilizing the amount of penetration of the end-sealing material when the injection hole is sealed, thereby to increase the reliability.
Although the injection hole of the LCD cell formed from a sealing material and end-sealing material of a UV-curing resin is employed, as described above, the end-sealing material and liquid crystal essentially are not compatible, and in particular, if they are used under elevated temperature and humid conditions, the charge retention of the liquid crystal deteriorates from the injection hole, and a picture quality problem such as white smear easily occurs. Generally, the mutual solubility of the sealing material with liquid crystal is also poor, and especially because the end-sealing material contacts with liquid crystal before it is cured, the pollution problem of liquid crystal is serious. The white smear is, for instance, a picture quality defect in which, in the normally white mode in which the liquid crystal becomes black when applied with a voltage, it remains white even if applied with a voltage.
However, in each publication described above, no reference is made to the problems with picture quality defects such as white smear. Further, for instance, the construction of a gap control member of the same material as the sealing material provided in the injection hole according to Published Unexamined Patent Applications No. 61-45225 is also effective to some degree against seepage of the pollution from the end-sealing material, but the sealing material is not compatible with liquid crystal as described above, and the sealing material itself as a gap control member may possibly pollute the liquid crystal. In addition, to reduce the contact area between the end-sealing material and the liquid crystal, it is possible to decrease the width of the injection hole, but it cannot be made small so much because of the pattern precision (position, width) of the sealing material. Even if the injection hole width can be made small, the increase in the liquid crystal injection time causes poor productivity and thus it lacks of feasibility. Further, it is possible to reduce the penetration of a pollutant by adversely affecting the mutual solubility between the liquid crystal and the end-sealing material, but this approach of deteriorating the mutual solubility has limitations by itself, if consideration is made to the adhesive power between glass and the end-sealing material in the sealed portion.
Further, a sufficient boding strength is kept between the sealing material and the glass substrate, or between the end-sealing material and the glass substrate, and there are less problems with the penetration of external impurities after the bonding. On the other hand, in the injection hole, the sealing material and the end-sealing material are bonded together to seal the liquid crystal, but the sealing material and the end-sealing material are both chemical materials, and the adhesiveness is not always sufficient. Further, if these are used under elevated temperature and humid conditions, they may cause a chemical reaction to emanate a pollutant from the bonded portion. Moreover, the water permeability in the bonded portion becomes high and the characteristics of the liquid crystal are easily deteriorated. The penetration of polluted liquid crystal from the bonded portion between the sealing material and the end-sealing material causes a picture quality defect such as white smear to easily occur.
The present invention was made to solve such technical problems, and its object is to prevent a pollutant seeped from the end-sealing material or the sealing material near the injection hole from penetrating into the pixel area, thereby to suppress the occurrence of a picture quality defect which is easily caused in the injection hole.
Further, another object is to reduce the contact area between the end-sealing material, which very likely deteriorates the liquid crystal, and the liquid crystal, by placing post structures in contact with or near the end-sealing material.
Still another object is to suppress the penetration of a pollutant seeped from the contact portion between the end-sealing material and the sealing material from penetrating into the pixel area.